Examples of quality of a synthetic quartz glass substrate include the size and density of defects on the substrate, flatness of the substrate, surface roughness of the substrate, photochemical stability of the substrate material, and chemical stability of the substrate surface. Requirements in regard of these qualities have been becoming severer, attendant on the trend toward higher precisions of the design rule. In a lithographic technology using an ArF laser light source with a wavelength of 193 nm and in a lithographic technology based on a combination of the ArF laser light source with an immersion technique, a silica glass substrate for a photomask is required to have good flatness. In this case, it is necessary to provide a glass substrate which not only shows a good flatness value simply but also has such a shape as to realize a flat exposure surface of the photomask at the time of exposure. In fact, if the exposure surface is not flat at the time of exposure, a shift of focus on the silicon wafer would be generated to worsen the pattern uniformity, making it impossible to form a fine pattern. Besides, the flatness of the substrate surface at the time of exposure that is required for the ArF immersion lithography is said to be not more than 250 nm.
Similarly, an EUV lithography in which a wavelength of 13.5 nm in the soft X-ray wavelength region is used as a light source has been being developed as a next-generation lithographic technology. In this technology, also, the surface of a reflection-type mask substrate is demanded to be remarkably flat. The flatness of the mask substrate surface required for the EUV lithography is said to be not more than 50 nm.
The current flatness-improving technique for silica glass substrates for photomasks is an extension of the traditional polishing technology, and the surface flatness which can substantially be realized is at best about 0.3 μm on average for 6025 substrates. Even if a substrate with a flatness of less than 0.3 μm could be obtained, the yield of such a substrate would necessarily be extremely low. The reason lies in that according to the conventional polishing technology, it is practically impossible to form recipes of flatness improvement based on the shapes of raw material substrates and to individually polish the substrates for improving the flatness, although it is possible to generally control the polishing rate over the whole surface of each substrate. Besides, for example, in the case of using a double side polishing machine of a batch processing type, it is extremely difficult to control the within-batch and batch-to-batch variations of flatness. On the other hand, in the case of using a single side polishing machine of a single wafer processing type, variations of flatness would arise from the shapes of the raw material substrates. In either case, therefore, it has been difficult to stably produce excellently flat substrates.
In the above-mentioned circumstances, a few processing methods aiming at improvement in surface flatness of glass substrates have been proposed. For instance, JP-A 2002-316835 (Patent Document 1) describes a method of improving the flatness of a surface substrate by applying local plasma etching to the substrate surface. In addition, JP-A 2006-08426 (Patent Document 2) describes a method of improving the flatness of a surface substrate by etching the substrate surface by use of a gas cluster ion beam. Further, US Patent Application 2002/0081943 A1 (Patent Document 3) proposes a method of improving the flatness of substrate surface by use of a polishing slurry containing a magnetic fluid.
In the cases of improving the flatness of a substrate surface by use of these novel technologies, however, there are such problems as large or intricate equipment and raised processing costs. For example, in the cases of plasma etching and gas cluster ion etching, the processing apparatus would be expensive and large in size, and many auxiliary equipments such as an etching gas supplying equipment, a vacuum chamber and a vacuum pump are needed. Even if the real processing time can be shortened, therefore, the total time taken for the intended improvement of flatness would be prolonged, taking into account the times taken for preparation for the processing, such as the rise times of the equipments, the time of drawing a vacuum, etc., and the times for pretreatment and post-treatment of the glass substrate. Furthermore, when depreciation expenses of the equipments and the costs of expendables, such as expensive gases (e.g., SF6) consumed in each run of processing, are passed onto the price of the mask-forming glass substrate, the improved-flatness substrate would necessarily be high in price. In the lithography industry, also, the substantial rise in the price of masks is deemed as a significant problem. Therefore, a rise in the price of the glass substrates for masks is undesirable.
In addition, JP-A 2004-29735 (Patent Document 4) proposes a substrate surface flatness-improving technology in which the pressure control means of a single side polishing machine is advanced and local pressing from the side of a backing pad is adopted to thereby control the surface shape of a substrate being processed. This flatness-improving technology is on the extension of an existing polishing technology, and is considered to be comparatively inexpensive to carry out. In this method, however, the pressing is from the back side of the substrate, so that the polishing action would not reach a protuberant portion of the face-side surface locally and effectively. Therefore, the substrate surface flatness obtained by this method is at best about 250 nm. Accordingly, the use of this flatness-improving method alone is insufficient in capability as a technology for producing a mask of the EUV lithography generation.